Interlocking piston button

ABSTRACT

An engine piston assembly includes a piston with two opposing boss bores, a first interlocking piston button with at least one interlocking protrusion compatible with at least one interlocking recess in the first boss bore, and a second interlocking piston button with at least one interlocking protrusion compatible with at least one interlocking recess in the second boss bore. The interlocking protrusions serve to prevent the buttons from rotating inside the boss bores and deforming an oil control ring inside an oil control ring groove of the piston assembly.

BACKGROUND

Many types of engines use one or more reciprocating pistons to generatepower. For example, an internal combustion engine utilizes combustion ofa fuel within combustion chambers to provide motive force tocorresponding pistons within the engine. Each piston includes rings thatprovide a fluid tight seal against a cylinder wall while the pistonmoves up and down within the cylinder. The fluid tight seal separatesthe combustion chamber from a crankcase of the engine. Each piston isconnected to a connecting rod using a wrist pin. The connecting rods areconnected to a crankshaft of the engine. Combustion of the fuel withinthe combustion chambers creates rapid gas expansion, which moves thepistons linearly within the cylinders. The connecting rods transfer thelinear motion of the pistons into rotational crankshaft motion, thusgenerating power at the crankshaft of the engine.

Several different options are available to retain a wrist pin in itsplace inside a piston. Clips can be used to clip the wrist pin in place,however, such clips are often difficult to change out quickly and can,in some cases, become dislodged during operation. For engines that arefrequently rebuilt, such as racing engines, the wrist pins need to beable to be replaced quickly. For those situations, buttons may be moreuseful. Buttons slide in and out of corresponding piston bosses easily,so they are easy to change quickly. However, in some racingapplications, pistons experience sufficient forces to cause circularbuttons to rotate within their corresponding piston bosses. Thisrotation can impinge on, and perhaps deform, an oil control ring insidean oil control ring groove of the piston, causing engine problems. Theengine may then be at a higher risk of failure due to damage to the oilcontrol ring and/or oil control ring groove. As a result, the engine mayneed to be disassembled more frequently in order repair any damagecaused by a damaged or deformed oil control ring and/or oil control ringgroove.

SUMMARY

The presently disclosed technology includes an engine piston assemblycomprising a piston including a first boss bore and a second boss bore,the first boss bore including a first interlocking recess and the secondboss bore including a second interlocking recess, wherein the secondboss bore opposes the first boss bore; a first piston button including afirst interlocking protrusion that fits within the first interlockingrecess of the first boss bore; and a second piston button including asecond interlocking protrusion that fits within the second interlockingrecess of the second boss bore.

The presently disclosed technology further includes a piston comprisinga first boss with a bore including two diametrically opposedinterlocking recesses; and a second boss with a bore including twoadditional diametrically opposed interlocking recesses, wherein thesecond boss bore is axially aligned with the first boss bore.

The presently disclosed technology still further includes a method ofassembling an engine piston comprising fitting a first piston buttonincluding a first interlocking protrusion into a first boss boreincluding a first interlocking recess; inserting a wrist pin through asecond boss bore and into a seat of the first piston button within thefirst boss bore; and fitting a second piston button including a secondinterlocking protrusion into the second boss bore including a secondinterlocking recess, while inserting the wrist pin into a seat of thesecond piston button.

This Summary is provided to introduce a selection of concepts in asimplified form that are further described below in the DetailedDescription. Other implementations are also described and recitedherein. This Summary is not intended to identify key features oressential features of the claimed subject matter, nor is it intended tobe used to limit the scope of the claimed subject matter. These andvarious other features and advantages will be apparent from a reading ofthe following detailed description.

BRIEF DESCRIPTIONS OF THE DRAWINGS

FIG. 1 is a perspective view of an exploded engine piston assemblyincluding interlocking piston buttons.

FIG. 2 is a perspective view of a piston for use in an engine pistonassembly including interlocking boss bores.

FIG. 3 is a perspective view of an interlocking piston button.

FIG. 4 shows example operations for assembling an engine pistonincluding interlocking piston buttons.

DETAILED DESCRIPTION

FIG. 1 is a perspective view of an exploded engine piston assembly 100including interlocking piston buttons 104, 105. The engine pistonassembly 100 includes a piston 102, which reciprocates within a cylinder106. The piston 102 includes two opposing bosses (e.g., boss 110), eachboss including a boss bore (e.g., boss bore 112). Interlocking pistonbutton 104 fits within boss bore 112 and interlocking piston button 105fits within the opposing boss bore (not shown) to retain a wrist pin 114between the interlocking piston buttons 104, 105. The wrist pin 114 isrotationally connected to a connecting rod 116. The piston 102reciprocates linearly within the cylinder 106, generating force bycompressing fluid in the cylinder 106, igniting the fluid, and capturingpower from the rapidly expanding combusting fluid. The connecting rod116 transmits linear force from the piston 102 to rotational force at acrankshaft (not shown), giving the engine power.

The interlocking piston buttons 104, 105 fit into the piston 102 to keepthe wrist pin 114 in place. To keep the wrist pin 114 from sliding outof the interlocking piston buttons 104, 105, the interlocking pistonbuttons 104, 105 each have a first inner diameter 118 and a second innerdiameter 119. In one implementation, the first inner diameter 118 of theinterlocking piston buttons 104, 105 is slightly larger than thediameter of the wrist pin 114. The diameter is measured so that thewrist pin 114 will fit inside of the interlocking piston buttons 104,105 with enough space to allow the wrist pin 114 to rotate within theinterlocking piston buttons 104, 105 (also referred to herein as aslip-fit wrist pin). In another implementation, the first inner diameter118 of the interlocking piston buttons 104, 105 is substantially thesame as the diameter of the wrist pin 114 so that a press-fit wrist pin114 will not rotate inside the interlocking piston buttons 104, 105.

The second inner diameter 119 of the interlocking piston buttons 104,105 is substantially smaller than the diameter of the wrist pin 114. Asa result, the wrist pin 114 is secured between the piston buttons 104,105 when assembled within the piston 102. The first inner diameter openstoward the center of the piston 102, while the second inner diameter 119passes through the entirety of each of the interlocking piston buttons104, 105. In other implementations, the first inner diameter and thesecond inner diameter may vary between the interlocking piston buttons104, 105. In still other implementations, the second inner diameter 119is omitted in one or both of the interlocking piston buttons 104, 105,thus creating a solid piston button surface facing the cylinder 106 wallwhen assembled.

The interlocking piston buttons 104, 105 also each have an outerdiameter 129 that is slightly smaller than the boss bore diameters. Thisallows the interlocking piston buttons 104, 105 to be slip-fit intotheir respective boss bores. In other implementations, the outerdiameters 129 are substantially the same as the boss bore diameters,thus permitting a press-fit connection between the interlocking pistonbuttons 104, 105 and their respective boss bores. The terminology“slightly smaller diameter” and “slightly larger diameter” as usedherein refers to a slip-fit connection with a standard tolerance forthat sort of connection. The terminology “substantially the samediameter” as used herein refers to a light interference fit,interference fit, or press-fit connection with a standard allowance forthose sorts of connections.

To prevent the interlocking piston buttons 104, 105 from rotating insidetheir respective boss bores, the interlocking piston buttons 104, 105each include interlocking protrusions 120, 121, 122, 123 that fits intocorresponding interlocking recesses (e.g., recesses 124, 125 in the bossbore 112). More specifically, interlocking protrusions 120, 121 ofpiston button 104 fit into interlocking recesses 124, 125 of boss bore112, respectively. Similarly, interlocking protrusions 122, 123 ofpiston button 105 fit into interlocking recesses (not shown) of theopposing boss bore 113. The interlocking boss bores and thecorresponding piston buttons 104, 105 may have a variety of differentsizes and shapes and may be in a variety of configurations, as discussedmore with reference to FIGS. 2 and 3, respectively. Further, while twointerlocking protrusions are shown on each of interlocking pistonbuttons 104, 105, in other implementations, any number of interlockingprotrusions may be used on each interlocking piston button.

To assemble the engine piston assembly 100, interlocking piston button104 is inserted into boss bore 112. The interlocking protrusions 120,121 on the interlocking piston button 104 fit into the interlockingrecesses 124, 125, respectively, located on the boss bore 112. Inanother implementation, where the interlocking piston button 104 hasonly one interlocking protrusion 120, only the interlocking protrusion120 fits into the interlocking recess 124. The wrist pin 114 is theninserted through the opposing boss bore and into the interlocking pistonbutton 104. The wrist pin 114 goes into the interlocking piston button104 until it is stopped by the second inner diameter 119 of theinterlocking piston button 104. Lastly, the interlocking piston button105 is inserted into the opposing boss bore, also inserting the wristpin 114 into the interlocking piston button 105. The interlockingprotrusions 122, 123 of the second interlocking piston button 105 fitinto the corresponding interlocking recesses of the second boss bore. Inanother implementation, where the interlocking piston button 105 hasonly one interlocking protrusion 122, only the interlocking protrusion122 fits into an interlocking recess of the second boss bore.Disassembly of the engine piston assembly 100 may be the opposite ofthat described above.

The piston 102 further includes an oil control ring groove 128. The oilcontrol ring groove 128 receives an oil control ring 130. The oilcontrol ring 130 scrapes oil from interior surfaces of the cylinder 106as the piston 102 reciprocates within the cylinder 106. The oil controlring groove 128 includes holes (not shown) so that the oil scraped fromthe cylinder 106 wall by the oil control ring 130 is distributed to aninterior of the piston 102, and vice versa. Some of the holes may gofrom the oil control ring groove 128 to the boss bores to lubricate thewrist pin 114, connecting rod 116, interlocking piston buttons 104, 105,and associated connections. The piston 102 further includes piston ringgrooves 134. The piston ring grooves 134 accommodate piston rings 133.The piston rings 133 provide a gas tight seal between the piston 102 andthe interior walls of the cylinder 106, thus separating a combustionchamber (not shown) from a crankcase (not shown) with a minimum of oilblow-by.

FIG. 2 is a perspective view of a piston 202 for use in an engine pistonassembly including interlocking boss bores 212, 213. More specifically,bosses 210, 211 include axially aligned interlocking boss bores 212,213, respectively. The interlocking boss bore 212 includes twointerlocking recess 224, 225. The interlocking boss bore 213 alsoincludes two additional interlocking recess (not shown). Theinterlocking boss bores 212, 213 are circular apertures that that extendthrough the bosses 210, 211, respectively. The interlocking boss bores212, 213 are further shown diametrically opposed across the piston 202in FIG. 2. The interlocking recesses 224, 225 extend from a perimeter ofthe circular boss bore 212 and diametrically opposed across the bossbore 212. The additional interlocking recesses also extend from aperimeter of the circular boss bore 213. The piston 202 further includesan oil control ring groove 228 with associated holes (e.g., hole 232)and piston ring grooves 234.

The interlocking recesses 224, 225 may be located at any radial positionabout a perimeter of the boss bore 212, provided that the interlockingrecesses 224, 225 do not intersect with the oil control ring groove 228.In the depicted implementation, interlocking recess 224 is located about180 degrees from interlocking recess 225 on the perimeter of theinterlocking boss bore 212. Additionally, the interlocking recesses 224,225 extend from the boss bore 212 in directions substantially parallelto a top of the piston 202. In another implementation, only oneinterlocking recess 224 is located at a bottom of the interlocking bossbore 212 extending from the boss bore 212 in a direction substantiallyperpendicular to the top of the piston 202. In yet anotherimplementation, additional interlocking recesses are located about theperimeter of the interlocking boss bore 212.

The interlocking recesses 224, 225 may also have a variety of profilesizes and shapes so long as they are of sufficient size to preventrotation of a corresponding interlocking button (not shown). In thedepicted implementation, the interlocking recesses 224, 225 in profileeach have a straight portion joined to a semi-circular portion. Thestraight portion is defined by two substantially parallel straightlines, joined to a circumference of the interlocking boss bore 212 by asmall outer radius. The interlocking recesses 224, 225 include radii,rather than sharp corners to reduce stress concentrations in the piston202 (i.e., they have a rounded profile). In another implementation, theinterlocking recesses 224, 225 have a straight portion joined by a curvewith a larger or smaller curve radius than the depicted semi-circle.Further, the interlocking recesses of boss bore 213 may be the same or adifferent size and shape from the interlocking recesses 224, 225 of bossbore 212.

The interlocking recesses 224, 225 in profile may also be shaped with nostraight portion. For example, in one implementation, the interlockingrecesses 224, 225 may be shaped by a semi-circular curve that intersectswith a perimeter of the interlocking boss bore 212. In anotherimplementation, the interlocking recesses 224, 225 may be shaped by acurve with a larger or smaller curve radius than a semi-circleintersecting with the perimeter of the interlocking boss bore 212.Additionally, the interlocking recesses 224, 225 may have differentshapes (e.g., triangular, square, etc.), with or without roundedcorners. The interlocking recesses 224, 225 may have a variety ofdepths, as long as the face of a corresponding interlocking button (notshown) is substantially coincident with the piston 202 wall when theinterlocking button is inserted into the interlocking boss bore 212.

Boss bore 213 also includes one or more interlocking recesses withvarious arrangements as described above with respect to interlockingrecesses 224, 225 of boss bore 212. Further, the interlocking recessesof boss bore 213 may be the same or different from the interlockingrecesses 224, 225 of boss bore 212. In various other implementations,the interlocking boss bores 212, 213 may each have any number ofinterlocking recesses so long as the interlocking recesses do notintersect with the oil control ring groove 228. The piston 202 may bemade of a variety of materials, including but not limited to iron,steel, aluminum, and various alloys thereof. The piston 202 furtherincludes passage 208, which may serve as an access port for removing theinterlocking button from the boss bore 212. The passage 208 may alsoserve to access a clip (not shown) that selectively secures theinterlocking button to a wrist pin (not shown, see wrist pin 114 of FIG.1).

FIG. 3 is a perspective view of an interlocking piston button 304. Theinterlocking piston button 304 has a first inner diameter 318, a secondinner diameter 319, and an outer diameter 329. In one implementation,the first inner diameter 318 of the interlocking piston button 304 isslightly larger than a diameter of a corresponding wrist pin (not shown)to allow for a slip-fit connection, which permits the wrist pin torotate with respect to the interlocking piston button 304. In anotherimplementation, the first inner diameter 318 of the interlocking pistonbutton 304 is substantially the same as the diameter of the wrist pin toallow for a press-fit wrist pin, which prevents the wrist pin fromrotating with respect to the interlocking piston button 304. The secondinner diameter 319 of the interlocking piston button 304 issubstantially smaller than the diameter of the wrist pin, so that thewrist pin is prevented from sliding through both ends of theinterlocking piston button 304. A combination of the first innerdiameter 318 and the second inner diameter 319 creates a seat 331 forthe wrist pin within the interlocking piston button 304.

The outer diameter of the interlocking piston button 304 is slightlysmaller than a diameter of a corresponding interlocking boss bore (notshown) so that the interlocking piston button 304 slip fits inside theinterlocking boss bore. In other implementations, the outer diameter ofthe interlocking piston button 304 is substantially the same as thediameter of the corresponding interlocking boss bore so that theinterlocking piston button 304 press fits inside the interlocking bossbore.

The interlocking piston button 304 includes interlocking protrusions320, 321. The interlocking protrusions 320, 321 may be located at anyradial position about a perimeter of the interlocking piston button 304.While two interlocking protrusions 320, 321 are shown in FIG. 3, anynumber of interlocking protrusions may be placed in any location aboutthe perimeter of the interlocking piston button 304 provided that theinterlocking protrusions do not intersect with an oil control ringgroove (not shown). In one implementation, the interlocking protrusion320 is located substantially 180 degrees from the interlockingprotrusion 321 about a circumferential perimeter of the interlockingpiston button 304. Further, both the interlocking protrusions 320, 321are oriented substantially coincident with a face 340 of the pistonbutton 304 (i.e., surface of the piston button 304 facing a cylinderwall when assembled).

The interlocking protrusions 320, 321 may also have a variety of profilesizes and shapes so long as they are of sufficient size to preventrotation within a corresponding interlocking boss bore (not shown). Inthe depicted implementation, the interlocking protrusions 320, 321 inprofile each have a straight portion joined to a semi-circular portion.The straight portion is defined by two substantially parallel straightlines, joined to a circumference of the interlocking piston button 304by a small outer radius. The interlocking protrusions 320, 321 includeradii, rather than sharp corners to reduce stress concentrations in thepiston button 304 (i.e., they have a rounded profile). In anotherimplementation, the interlocking protrusions 320, 321 have a straightportion joined by a curve with a larger or smaller curve radius than thedepicted semi-circle.

The interlocking protrusions 320, 321 in profile may also be shaped withno straight portion. For example, in one implementation, theinterlocking protrusions 320, 321 may be shaped by a semi-circular curvethat intersects with a perimeter of the interlocking piston button 304.In another implementation, the interlocking protrusions 320, 321 may beshaped by a curve with a larger or smaller curve radius than asemi-circle intersecting with the perimeter of the interlocking pistonbutton 304. Additionally, the interlocking protrusions 320, 321 may havedifferent shapes (e.g., triangular, square, etc.), with or withoutrounded corners. The interlocking protrusions 320, 321 may have avariety of depths, as long as the face of the interlocking piston button304 is substantially coplanar with a piston wall (not shown) when theinterlocking piston button 304 is inserted into an interlocking bossbore.

Multiple interlocking piston buttons may include one or moreinterlocking recesses with various arrangements as described above withrespect to interlocking protrusions 320, 321 of interlocking pistonbutton 304. Further, the interlocking recesses of other interlockingpiston buttons may be the same or different from the interlockingprotrusions 320, 321 of interlocking piston button 304. In various otherimplementations, the interlocking piston buttons may each have anynumber of interlocking protrusions so long as the interlockingprotrusions do not intersect with a corresponding oil control ringgroove in a piston. The interlocking piston button 304 may be made of avariety of materials, including but not limited to iron, steel,aluminum, and various alloys thereof.

The interlocking piston button 304 includes an indentation 336 in theface 340 of the piston button 304. The piston button 304 is positionedto align with an oil control ring groove (not shown, see oil controlgroove 228 of FIG. 2) when installed in a piston. The indentation 336accommodates an oil control ring by matching the surrounding oil controlring groove of the piston, as illustrated in FIG. 1. Size and locationof the indentation 336 on the interlocking piston button 304 is definedby the size and location of the oil control ring groove on the piston.In one implementation, the indentation 336 extends from a top edge ofthe face 340 of the interlocking piston button 304. In anotherimplementation, the indentation 336 may remove a portion of the face 340of the interlocking piston button 304 within a middle portion of theinterlocking piston button 304, forming a channel (not shown). A widthof the channel would substantially match a width of the oil control ringgroove. A depth of the indentation 336 is also defined by the oilcontrol ring groove. In various implementations, the depth of theindentation 336 is the same as the depth of the oil control ring groove,so that the indentation is coincident with the oil control ring when theinterlocking piston button is inserted into the piston.

FIG. 4 shows example operations 400 for assembling an engine pistonincluding interlocking piston buttons. A first fitting operation 405fits a first interlocking piston button into a corresponding first bossbore in the engine piston. The first piston button includes one or moreinterlocking protrusions that correspond to one or more interlockingrecesses in the first boss bore. When the first piston button isinserted into the first boss bore, the interlocking protrusion(s) fitinto the interlocking recess(es), which prevents the first piston buttonfrom rotating within the first boss bore. Further, the first pistonbutton may include an indentation that aligns with an oil control ringgroove when fit within the piston. The indentation may permit an oilcontrol ring to extend across the first piston button withoutinterruption. Still further, since the first piston button is preventedfrom rotating within the first boss bore, potential deformation of theoil control ring is also prevented.

A first inserting operation 410 inserts a wrist pin into the firstinterlocking piston button. More specifically, the wrist pin is insertedthrough a second boss bore, and inserted into a seat within the firstinterlocking piston button in the first boss bore. In someimplementations, before the wrist pin is inserted into the firstinterlocking piston button, it will also be inserted through aconnecting rod.

A second fitting operation 415 fits the second interlocking pistonbutton into the second corresponding boss bore in the engine piston. Thesecond piston button includes one or more interlocking protrusions thatcorrespond to one or more interlocking recesses in the second boss bore.When the second piston button is inserted into the second boss bore, theinterlocking protrusion(s) fit into the interlocking recess(es), whichprevents the second piston button from rotating within the second bossbore. Further, the second piston button may include an indentation thatalso aligns with the oil control ring groove when fit within the piston.The indentation may permit the oil control ring to extend across thesecond piston button as well without interruption. Still further, sincethe second piston button is prevented from rotating within the secondboss bore, potential deformation of the oil control ring is alsoprevented. As the second piston button is fit into the second boss bore,the wrist pin is inserted into a seat on the second piston button. As aresult, the opposing first and second piston buttons hold the wrist pinin place within the engine piston.

The above specification, examples, and data provide a completedescription of the structure and use of exemplary embodiments of theinvention. Since many embodiments of the invention can be made withoutdeparting from the spirit and scope of the invention, the inventionresides in the claims hereinafter appended. Furthermore, structuralfeatures of the different embodiments may be combined in yet anotherembodiment without departing from the recited claims.

What is claimed is:
 1. An engine piston assembly comprising: a pistonincluding an oil control ring groove, a first boss bore, and a secondboss bore, the first boss bore including a first interlocking recess andthe second boss bore including a second interlocking recess, wherein thesecond boss bore opposes the first boss bore; a first piston buttonincluding a first interlocking protrusion that fits within the firstinterlocking recess of the first boss bore and a first indentation thataligns with the oil control ring groove when fit within the piston; anda second piston button including a second interlocking protrusion thatfits within the second interlocking recess of the second boss bore and asecond indentation that aligns with the oil control ring groove when fitwithin the piston.
 2. The engine piston assembly of claim 1, wherein thefirst piston button is slip-fit within the first interlocking recess andthe second piston button is slip-fit within the second interlockingrecess.
 3. The engine piston assembly of claim 1, wherein the first bossbore includes a third interlocking recess, the first piston buttonincludes a third interlocking protrusion, and the third interlockingprotrusion fits within the third interlocking recess of the first bossbore.
 4. The engine piston assembly of claim 3, wherein the second bossbore includes a fourth interlocking recess, the second piston buttonincludes a fourth interlocking protrusion, and the fourth interlockingprotrusion fits within the fourth interlocking recess of the second bossbore.
 5. The engine piston assembly of claim 1, wherein one or both ofthe first piston button and the second piston button include a firstinner diameter and a second smaller inner diameter forming a seat. 6.The engine piston assembly of claim 5, further comprising: a wrist pinslip-fit between the first piston button and the second piston buttonand resting in the seat.
 7. The engine piston assembly of claim 1,wherein a depth of the first interlocking recess is substantially thesame as the depth of the first interlocking protrusion and a depth ofthe second interlocking recess is substantially the same as the depth ofthe second interlocking protrusion.
 8. The engine piston assembly ofclaim 1, wherein one or both of the first and second interlockingprotrusions have a rounded profile.
 9. A piston comprising: a first bosswith a bore including two diametrically opposed interlocking recesses; asecond boss with a bore including two additional diametrically opposedinterlocking recesses, wherein the second boss bore is axially alignedwith the first boss bore; and an oil control ring groove, the oilcontrol ring groove intersecting the first boss bore and the second bossbore.
 10. The piston of claim 9, further comprising: a first pistonbutton including two diametrically opposed interlocking protrusions thatfit within the two diametrically opposed interlocking recesses of thefirst boss bore; and a second piston button including two additionaldiametrically opposed interlocking protrusions that fit within the twodiametrically opposed interlocking recesses of the second boss bore. 11.The piston of claim 10, wherein one or both of the first piston buttonand the second piston button includes an indentation that aligns withthe oil control ring groove when fit within the piston.
 12. The pistonof claim 10, wherein one or both of the first piston button and thesecond piston button include a first inner diameter and a second smallerinner diameter forming a seat.
 13. The piston of claim 12, furthercomprising: a wrist pin slip-fit between the first piston button and thesecond piston button and resting in the seat.
 14. The piston of claim 9,wherein one or more of the interlocking recesses have a rounded profile.15. A method of assembling an engine piston comprising: fitting a firstpiston button including a first interlocking protrusion into a firstboss bore including a first interlocking recess such that a firstindentation in the first piston button aligns with an oil control ringgroove in the engine piston; inserting a wrist pin through a second bossbore and into a seat of the first piston button within the first bossbore; and fitting a second piston button including a second interlockingprotrusion into the second boss bore including a second interlockingrecess such that a second indentation in the second piston button alignswith the oil control ring groove, while inserting the wrist pin into aseat of the second piston button.
 16. The method of claim 15, whereinfitting the first piston button into the first boss bore includesfitting the first interlocking protrusion into the first interlockingrecess, and wherein fitting the second piston button into the secondboss bore includes fitting the second interlocking protrusion into thesecond interlocking recess.
 17. The method of claim 15, wherein thefirst boss bore includes a third interlocking recess, the first pistonbutton includes a third interlocking protrusion, and wherein fitting thefirst piston button into the first boss bore includes fitting the thirdinterlocking protrusion into the third interlocking recess.
 18. Themethod of claim 15, wherein the second boss bore includes a fourthinterlocking recess, the second piston button includes a fourthinterlocking protrusion, and wherein fitting the second piston buttoninto second first boss bore includes fitting the fourth interlockingprotrusion into the fourth interlocking recess.
 19. The method of claim15, wherein one or both of the first and second interlocking protrusionshave a rounded profile.